Method and apparatus for excluding silt from a dredging operation

ABSTRACT

By measuring pressures in the particulate solid material surrounding a suction pipe at known points spaced along the pipe, sand of one known specific gravity which is covered by a layer of silt of another specific gravity may be dredged to the exclusion of the silt. Due to the difference in specific gravities, the approximate location of the interface between the sand and silt may be monitored and the lower end of the pipe moved so as to maintain it below such interface at all times during the dredging operation.

175-50. R 02-29-72 XR 3,645,018

uuucu utamuu 1. "inll 1 3,645,018

De Koning et al. 1 Feb. 29, 1972 [54] METHOD AND APPARATUS FOR 934,031 9/1909 Askew ..37/58 EXCLUDING SILT FROM A'DREDGING g-g l 911952 Penningmnm TION 1,550 12 1953 Graham ....37/58 OPERA 3,224,121 12/1965 Denning.... ...37/58 [72] Inventors: Jan De Koning, Soetendaal 20, Amster- 3,262,317 7/1966 Smith, Jrm ..73/407 dam, Romke van der Veen, 21 Prof. Dr. 3,369,395 2/1968 Scott et al.. ..73/152 Hesselaan, Jutphaas, both of Netherlands 3,373,604 3/1968 Dudman ..73/152 3,422,672 l/l969 Payne ..73/151 [221 Mar-28,1969 3,427,885 -2/1969 Konigsberg ..73/407 x N e [211 App] 81 L396 FOREIGN PATENTS OR APPLICATIONS 6,501,405 s/mse Netherlands ..37/58 [63] Continuation-impart of Ser. No. 729,229, May 15,

19 g Primary Examiner-William 8. Penn v Assistant ExaminerClifford D. Crowder [30] Foreign Application Priority Data Attorney-Imirie; Smiley, Snyder and Butrum Ami-8,1968 Netherlands ..6804953 [57 4 I ABSTRACT 52 US. Cl ..37/58, 37/195 73/151, BY measuring Pressures Pamiculate material 73/407 /50 rounding a suction pipe at known points spaced along the [51] Int. Cl. ..EilZf 3/88 pipe Sand of one known Specific gravity which is covered by a [58] Field of search ..73/407 151 152- 37/58 'ayemfsilmf amber SPecific gravity may be d'edged 6 exclusion of the silt. Due to the difference in specific gravities, the approximate location of the interface between the sand [56] Reerences cited and silt may be monitored and the lower end of the pipe moved so as to maintain it below such interface at all times UNITED STATES PATENTS during the dredging Operation- 3,263,615 8/1966 l-lofer ..37/58 UX 7 Claims, 4 Drawing Figures F'IGJ F a x IN VENTOR 3 ATTORNEY};

Patented Feb. 29, 1972 lllillillllu liilnvvll l.

will-l Ill-ill llllllllalln'im JAN ae KONING goM/(E vam der VEEN This application is a continuation-in-part of application Ser. No. 729,229, filed May 15, 1968.

The invention relates to a method of suction dredging sand from a body of sand covered by a layer of other kind of soil, wherein a suction pipe is inserted through said layer into said body of sand, said suction pipe is subjected to a source of suction and is communicated with said body of sand for introducing sand into said suction pipe.

When holding the lower end of a suction pipe at a fixed point within said body of sand during sucking up sand, all soil found above said fixed point will be sucked up after a while, resulting in sucking up silt of said layer covering the sand. Particularly when the layer of silt covering the sand is thick, the quantity of sand found above the suction mouth is unknown. This is a problem particularly owing to the fact that silt from surrounding places fiows into the cavity, which during the sucking up operation arises above the suction mouth. The invention provides an improved method of the kind mentioned above, wherein the effect of a physical property of the soil is measured at at leastone level having a determined vertical distance above the level of sand introduction into said suction pipe.

Preferably, a physical characteristic such as pressure effect due to a physical property such as specific gravity of the soil is measured at at least one level having a determined vertical distance to the level of sand introduction into said suction pipe. If the specific gravity of the soil found above the suction mouth is known the kind of soil can be deduced from this pressure measurement so that it may be determined whether or not it is necessary to lower the suction mouth in the sand or to displace the suction mouth to prevent sucking up of the undesired silt.

The invention also provides a suction dredger installation comprising a suction pipe which is connected to a pump and having a suction mouth. In order to adapt it for performing the method according to the invention this suction dredger installation is provided with a measuring device for measuring the effect of a physical property of the soil found above the suction mouth.

The above mentioned and other features of the invention will be elucidated in the following description of a number of preferred embodiments of methods and suction dredger installations according to the invention.

In the drawing:

FIG. 1 is a diagrammatic illustration of a suction dredger installation during sucking up sand, and

FIGS. 2, 3 and 4 illustrate the lower ends of modified forms of suction pipes.

The suction dredger installation according to the invention mainly consists of a vessel 10, a pump 11, a pressure line 12 connected to the pressure side of the pump 11 and a suction pipe 13 which has a suction mouth 20 and is connected to the suction side of the pump 11.

In the embodiment illustrated the pump 11 is disposed at the water level 14 and the suction pipe 13 is mounted swingably around the center line of the pump 11.

FIG. 1 shows (counting upwards) a layer of sand 15, a layer of clay l6, and a layer of water 18. The free end of the suction pipe 13 is inserted into the soil to a great depth, for instance 20 m. The soil shown enclosed by a chain-dot line 19 and surrounding the end of the suction pipe 13 is brought into suspension by underdigging. Outside the line 19, the soil is in its normal undisturbed condition.

If during the sucking up of sand, the quantity of sand brought into suspension is known, which sand is still above the suction mouth 20, this quantity can be taken into account during the sucking up of sand and the displacement of the suction pipe 13.

To this end, the suction dredger installation shown in FIG. 1 has a measuring device mainly consisting of two pressure pickups 6 and 5 located at the measuring points 8 and 9 at the end of the suction pipe 13, and an indicator 22. The pressure pickup 6 measures the pressure prevailing at measuring point 8 formed by a column of sand 2,, a column of silt K and a column of water W,,. The pressure pickup 5 measures the pressure prevailing at measuring point 9 formed by a column of silt k, and a column of water W..

The indicator 22 consists of a pointer 23 which is attached to the suction pipe 13 and has a graduated scale 24 from which the angle a can be read off, which angle is enclosed by the suction pipe 13 and the horizontal plane. The depths D and D, of the pressure pickups 6 and 5 respectively below the surface 14 of the water are equal to I; sin a and 1, sin a respectively wherein l and 1 respectively represent the length of the suction pipe from the pump 11 to. the pickups 6 and 5 respectively.

Accepting the approximation that W W, the vertical distance x of the silt to the suction mouth can be calculated by the equations:

P and P are the pressures measured at the measuring points 8 and 9 respectively,

W and W, are the water columns above the measuring points 8 and 9 respectively,

K and K, are the silt columns above the measuring points 8 and 9 respectively,

S, is the specific gravity of the silt, and l S, is the specific gravity of the sand.

From the two above equations there follows:

Kg Kg l sin a-Z from which P3 Pg S l sin n+2; (S=S Further The specific gravity of the silt S and the specific gravity of the drifting sand are measured in advance I by means of drillings, if those values might not be known. The pressures P and P are measured by means of the pressure pickups 6 and 5 respectively.

The pressures P and P each reduced with the water column W can be measured considerably more simply and accurately by means of pressure-difference pickups 25 and 21 known per se which are applied to the suction pipe 45 illustrated in FIG. 2. In each of the pressure-difference pickups 25 and 21, one pressure chamber 28 is acted upon by the local pressure in the soil whereas the other pressure chamber 26 is connected to the water 18 above the soil via a line 27. The pressure chamber 28 is separated from the soil by a diaphragm 17 for protecting the measuring diaphragm 44 from damage.

The suction pipe 38 shown in FIG. 3 has a measuring system formed by a pressure water line 29 which is connected to a pressure source and branches into two narrow branches 30, 31 which have an outflow aperture 32 in the soil at measuring point 8 adjacent the suction mouth 34 and an outflow aperture 33 at a higher level in the silt at the above mentioned measuring point 9 respectively. Adjacent the outflow apertures 32, 33 the branches 30, 31 are connected to either side of the pressurerdifference pickup 37 via measuring lines 35 and 36 respectively. This pressure-difference pickup 37 having a measuring membrane 3 measures in this way the difference in pressure between the apertures 32 and 33. The column of sand x above the suction mouth 32 can be derived therefrom.

The measuring device is very useful for timely displacing or adjusting the suction mouth at the required depth. Of course the suction pipe can be inserted so deeply into the sand that the measuring point 9 is also in the sand. As long as the measuring point 9 stayssubmerged in the sand a pressure difference S 1 sin a is measured. As soon as the silt lowers past measuring point 9, this pressure difference decreases, from which it can be deduced that the mouth'20 has to be displaced after some time for preventing sucking up silt.

The aperture 32 and the pressure pickups 6 and 25 respectively are preferably not disposed in exactly the same place as the suction mouth, but at a small distance m thereabove, where the flows around the suction mouth have little effect, if any, on the pressure measured.

The pressure pickups 5 and 6 and the pressure-difference pickups 21, 25 and 37 are, for instance, of the known electric type and are each connected via electric wires to an indicating system (not shown) disposed on a control panel on board the vessel 10.

The suction pipe 38 shown in FIG. 3 comprises a pipe 39 connected to the pump 11 and a sliding pipe 40, which can be moved axially in the pipe 39 by means of a hydraulic ram 41. An external jacket 42 is disposed around the pipe 39 and the sliding pipe 40, which jacket is attached to the pipe 39 and is open at its upper-side to be in communication with the water 18. By reason of the fact that no or very little water penetrates into the soil, a heavy suspension of drifting sand consisting of a large quantity of sand with little water enters through the suction mouth 34 disposed at the lower end of the outer jacket 42. In proportion as the telescopic pipe 40 is drawn upwards, a larger quantity of water comes into the sliding pipe 40 via the outer jacket 42 along the lower edge 43 of the telescopic pipe 40, and is also sucked up. On the other hand the sand rises higher in the outer jacket 42 in proportion as the jacket 42 is inserted more deeply into the soil. The concentration of sand in the sucked-up suspension can now be regulated by adjusting the distance of the edge 43 above the suction mouth 34 in dependence on the weight of the column of soil standing above the. suction mouth 34. To effect this regulation, a pickup 4 at the measuring point 7 which is above the soil and in the water is provided besides a pick up at each measuring point 8 and 9. From the pressure difference between this pressure pickup 4 and the pressure pickup 6 at measuring point 8, the total weight of the column of soil and thus the depth of the suction mouth under the ground surface can be derived.

The pressure-difference pickups t to t inclusive are provided in the modification of FIG. 4, each picking up a pressure difference in the soil between a pair of the measuring points 47 to 57 inclusive which are equidistantly spaced apart from each other.

The picked up values are read ofi from indicators (not shown) positioned on board of the vessel or successively in turn on one and the same indicator (not shown). From these pressure differences the contact surface between the silt and the sand can be followed very well.

it should be noted that the vertical distance between the silt and the suction mouth is preferably determined by means of pressure pickups or pressure-difference pickups. It is no impossible, however, that other pickup may be provided, which instead of measuring a physical characteristic associated with difference in specific gravity respond to physical characteristics associated with other differences in physical properties of silt and sand, for instance difference of viscosity, the transmission factor of rays or of electric resistance.

The distance between the pickups 5 and 6 of FIG. 1 is, for instance, about 5 m. The distance between the outflow apertures 32 and 33 of FIG. 3 is, for instance, 3 m.

The distance between each pair of the method pressures 47 to 57 inclusive is, for instance, k m.

The length of each of the above-mentioned suction pipes is, for instance, 50 m. whereas the lowest measuring point 8 and the lowest measuring point 47 is spaced a distance m of, for instance, l m.

What we claim is:

l. A method of suction dredging sand which comprises the steps of:

a..inserting the lower end of a suction pipe into an underwater body of sand having a known physical property and which body of sand lies below a layer of silt having a known physical property whose value is different from that of the corresponding physical property of the sand,

and which silt is to be excluded from recovery; b. maintaining suction on the suction pipe to withdraw a suspension of sand in water through said lower end of the suction pipe and deliver it to a predetermined region;

0. during step (b), measuring a physical characteristic associated with said known physical property of the material outside said suction pipe and above said lower end of the suction pipe;

d. determining from the measurement of step (c) the relative position of said lower end of the suction pipe with respect to the interface between the silt layer and the body of sand; and

e. maintaining the lower end of the suction pipe in dependence upon the determination of step (d) so as to remain below said interface during step (b) to prevent sucking up silt.

2. The method according to claim 1 wherein the specific gravities of said sand and said silt are known, and pressure is the physical characteristic measured in step (c).

3. The method according to claim 1 wherein step (c) comprises measuring said physical characteristic of the silt layer at a first predetermined point and measuring said physical characteristic of said body of sand at a second predetermined point.

4. The method according to claim 3 wherein the specific gravities of said sand and said silt are known, and pressure is the physical characteristic measured in step (c).

5. A suction dredger assembly comprising, in combination;

a fluid pump having a suction inlet and a pressure outlet;

a suction pipe assembly having one end connected to said suction inlet of the pump and having an opposite, lower end for inducting a suspension of sand in water therethrough;

means for positioning said lower end of the suction pipe assembly within an underwater body of sand having a known physical property and which body of sand lies below and presents an interface with a layer of silt wherein said physical property is also known and whose value is different from that of said sand and which silt is to be excluded from recovery by the dredger assembly; and

measuring means supported on said suction pipe assembly in predetermined spaced relation to said lower end thereof for producing an output signal which is proportional to a physical characteristic associated with said known physical property at the location of said measuring means whereby the relative position of said interface with respect to said lower end of the suction pipe may be ascertained so that said means for positioning may be operated to maintain said lower end of the suction pipe below said interface while suspension is being sucked up.

6. The suction dredger assembly as defined in claim 5 wherein said measuring means comprises a plurality of pressure sensing elements, two of which are disposed at said predetermined points and others of which are disposed at other points spaced predetermined distances from said lower end of the suction pipe assembly.

7. The suction dredger assembly as defined in claim 5 wherein said measuring means comprises a pair of pressure sensing elements one of which is disposed at one selected point and the other of which is disposed at another selected point, said points being spaced from each other and from said lower end of the suction pipe assembly. 

1. A method of suction dredging sand which comprises the steps of: a. inserting the lower end of a suction pipe into an underwater body of sand having a known physical property and which body of sand lies below a layer of silt having a known physical property whose value is different from that of the corresponding physical property of the sand, and which silt is to be excluded from recovery; b. maintaining suction on the suction pipe to withdraw a suspension of sand in water through said lower end of the suction pipe and deliver it to a predetermined region; c. during step (b), measuring a physical characteristic associated with said known physical property of the material outside said suction pipe and above said lower end of the suction pipe; d. determining from the measurement of step (c) the relative position of said lower end of the suction pipe with respect to the interface between the silt layer and the body of sand; and e. maintaining the lower end of the suction pipe in dependence upon the determination of step (d) so as to remain below said interface during step (b) to prevent sucking up silt.
 2. The method according to claim 1 wherein the specific gravities of said sand and said silt are known, and pressure is the physical characteristic measured in step (c).
 3. The method according to claim 1 wherein step (c) comprises measuring said physical characteristic of the silt layer at a first predetermined point and measuring said physical characteristic of said body of sand at a second predetermined point.
 4. The method according to claim 3 wherein the specific gravities of said sand and said silt are known, and pressure is the physical characteristic measured in step (c).
 5. A suction dredger assembly comprising, in combination; a fluid pump having a suction inlet and a pressure outlet; a suction pipe assembly having one end connected to said suction inlet of the pump and having an opposite, lower end for inducting a suspension of sand in water therethrough; means for positioning said lower end of the suction pipe assembly within an underwater body of sand having a known physical property and which body of sand lies below and presents an interface with a layer of silt wherein said physical property is also known and whose value is different from that of said sand and which silt is to be excluded from recOvery by the dredger assembly; and measuring means supported on said suction pipe assembly in predetermined spaced relation to said lower end thereof for producing an output signal which is proportional to a physical characteristic associated with said known physical property at the location of said measuring means whereby the relative position of said interface with respect to said lower end of the suction pipe may be ascertained so that said means for positioning may be operated to maintain said lower end of the suction pipe below said interface while suspension is being sucked up.
 6. The suction dredger assembly as defined in claim 5 wherein said measuring means comprises a plurality of pressure sensing elements, two of which are disposed at said predetermined points and others of which are disposed at other points spaced predetermined distances from said lower end of the suction pipe assembly.
 7. The suction dredger assembly as defined in claim 5 wherein said measuring means comprises a pair of pressure sensing elements one of which is disposed at one selected point and the other of which is disposed at another selected point, said points being spaced from each other and from said lower end of the suction pipe assembly. 